Ferrimagnetic crystals



April 1963 M. TANENBAUM 3,085,981

FERRIMAGNETIC CRYSTALS Filed March 25, 1960 F/G. FIG. 2

IN VE' N TOR By M. TANE NBA UM ATT'ORN V 3,085,981 FERRIMAGNETICCRYSTALS Morris Tanenbaurn, Madison, N.J., assignor to Dell TelephoneLaboratories, Incorporated, New York, N.Y., a corporation of New YorkFiled Mar. 25, 1960, Ser. No. 17,539 1 Claim. (Cl. 252-625) It has beenfound that the garnet structures being.

grown today are of such quality that the resonance characteristics, forinstance, the resonance line width, are limited by the degree of surfacesmoothness. Considering a perfect sphere of single crystal yttrium-irongarnet, the relations between the frequency v for resonance and thedirect-current magnetic field H for such a sphere is given by thefollowing equation:

where v is in megacycles per second and H is in oersteds. The line widthof the resonance expressed by the change in magnetic field required forthe absorption to fall to one-half of its maximum value will be a fewtenths of an oersted at room temperature. Such narrow line widths havebeen observed in single crystals of yttrium-iron garnet at frequenciesin the neighborhood of 5,000 megacycles per second. However, thesenarrow ideal lines can only be obtained by the most careful polishing ofthe small spheres. These spheres are generally about 15 mils in thediameter and must be polished approximately one week with successivegrades of abrasive material until they achieve a sphericity and surfacefinish which is adequate. Even after this laborious and expensive polishing, careful examination of the surface of these spheres indicates thatsmall pits and scratches still exist and are contributing to themeasured resonance line width. These effects occur because of thedemagnetizing fields which are produced when the arrays of magneticdipoles in the sphere reach an abrupt discontinuity at the imperfectsurface. Typical resonance line widths of garnets before and aftervarious grades of polishing have been studied by Spencer, LeCraw andPorter and are reported at pages 1311 through 1313, The Physical Review,volume 110, No. 6, June 15, 1958.

The theory and measurement of resonance line widths can be found inFerrites by J. Smit and H. P. I. Wijn.

An object of the present invention, therefore, is to provide a crystalstructure in which the demagnetizing effects of minute surfaceimperfections, left even as a result of extensive physical polishing,are rendered less effective.

The present invention proposes to diffuse into the outer surface of thecrystal a nonmagnetic ion which will substitute for the iron ion in thegarnet lattice. The resulting crystal will then have an outer shellregion of nonmagnetic garnet structure with a ferrirnagnetic interiorregion. The interface between these regions exhibits a continuity orsmoothness far superior to that of the original surface.

This may perhaps be best understood when considered in conjunction withthe drawing in which:

United States Patent 7 3,085,981 Patented Apr-.16, 1963 ice ' polishing;

FIG. 2 is a sectional view of a portion of the same crystal during thefirst phase of the treatment of the invention;

FIG. 3 is a sectional view of a portion of the same crystal at asubsequent stage of the operation;

FIG. 4 is a sectional view of a portion of the desired finished crystal;and

FIG. 5 is a schematic view of an appropriate apparatus used inobtainingthe desired crystal.

Referring to FIGS. 1 through 4 which show the crystal in various stagesof treatment according to the invention, numeral 1 refers to theferrimagnetic interior layer and 2 denotes the crystal surface. FIGS. 2through 4 show the diffusion layer ofnonmagnetic material 3, and theinterface betweenthe magnetic and nonmagnetic regions 4.

The critical interface ordinarily impairing the resonant properties isthe ferrimagnetic surface denoted Z in FIG. 1. In FIGS. 2 through 4 theoriginal crystal surface 2 is now nonmagnetic and the irregularitiesthere will not affect the magnetic dipoles; The critical interface thenbecomes interface 4, that is, the interface between the magnetic andnonmagnetic regions. As is seen, this diffusion layer interface, thatis, the interface between the magnetic region 1 and the nonmagneticregion 3 will initially reflect the surface imperfections. However, thedeeper the layer diffuses between preferred depths of 1-50 micronsdepending upon the depth of the initial irregularities, the lesspronounced these irregularities become. This is apparent from anexamination of the changing character of the interface 4 in FIGS. 2through 4.

FIG. 4 then shows the finished crystal 15 mils in diameter having anappropriate diffusion layer of the order of 15 microns. The interfacethere appears substantially smooth. Hence, the magnetic dipoles leavingthe ferrimagnetic region will emerge through the spherical surfacehaving a smoothness which will not significantly upset their resonanceproperties.

The diffusion procedure makes use of an oxide source of a group 3 metal,typically Ga or A1. The group 3 metal upon diffusing into the irongarnet replaces the magnetic iron ion in the lattice forming anonmagnetic garnet such as Y Ga O The source is conveniently a mixtureof the group 3 metal oxide and the free group 3 metal. These materialsreact to form a suboxide of the group 3 metal which is sufficientlyvolatile and readily diffuses into the iron garnet placed adjacent thesource. With respect to gallium the reactions appear thus:

The following specific embodiment is given to illustrate one particularprocedure for practicing the invention. The apparatus used is a typicaltwo-zone furnace as is well known in the art and shown schematically inFIG. 5. A quartz tube 11 is evacuated to approximately 10- millimetersof mercury. A mixture of 1 gram of powdered gallium oxide is intimatelymixed with 2 grams gallium metal, the metal being liquid at roomtemperature, and the mixture, contained in the silica boat 12, is placedin the first zone 13. This zone, heated by coil 14 which controls theatmosphere over the Ga, Ga O system, is heated to 700 C. A ferrimagneticgarnet crystal was coarsely polished with an abrasive of 3 microns meangrit size and consequently exhibited surface characteristics similar tothose of FIG. 1, having an average depth of 3 microns. The crystal 15,15 mils in diameter and having the composition Y3Fe5012, is placed inthe tube at zone 16. The garnet before treatment has a resonance linewidth of approximately 2 oersteds. The zone 16, controlled thermally bycoil 17, is heated to 1200 C. These conditions are maintained for aperiod of 24 hours. This two-zone diflusion technique is known to theart and is treated by Frosch and Derrick in the Journal of theElectrochemical Society, volume 105, December 1958, at pages 695 through699.

Upon completion of the required period, .the garnet crystal exhibits adiffusion layer of nonmagnetic material to a depth of approximately 15microns and shows an interface between the non-magnetic diffused layerand the remaining ferrimagnetic portions of the crystal which has agreater degree of uniformity than the surface of the crystal. Thissmoother interface is shown in the cross-section of FIG. 4. Theresonance line width of this crystal after treatment according to theinvention is .4 oersteds or a reduction by a factor of 5.

This procedure is given as exemplary only and is not intended aslimiting the invention. Hence, any ferrimagnetic garnet may be sotreated to reduce the adverse fer-rimagnetic effects of surfaceimperfections. These encompass yttrium-iron garnets and rare earth irongarnets. The nonmagnetic ion to be diffused into the garnet lattice isrestricted by the size of the atom. Gallium and aluminum areappropriate.

The process conditions vary according to the materials and systemsemployed, but generally, temperatures between 500 C. and 1000 C. in zone1 and 900 C. to 1500 C. in zone 2 are proper. The atmosphere in thequartz tube is preferably a vacuum. Alternatively, an inert atmospheresuch as argon gas may be used. While the invention has been described interms of spherical crystals it is obvious to one skilled in the art thatvarious other shapes such as rods or disks may be advantageously treatedin this manner.

What is claimed is:

The method of treating a single crystal yttrium iron garnet to provide asmooth magnetic surface which comprises diffusing into the surface ofsaid garnet an ion selected from the group consisting of aluminum andgallium whereby the diffusion layer is rendered non-magnetic and theferrimagnetic surface exhibits a greater degree of uniformity than thesurface of the garnet crystal.

Pauthenet: Supplement to J. of Applied Physics, vol. 30, No. 4, pages290S292,S, April 1959.

